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The Doppler Effect is an interesting phenomenon that occurs when an object producing sound is moved relatively to the listener.

Consider the following: When a car blaring its horn is behind you, the pitch is higher as it is approaching, and becomes lower as it is moving away. This is only noticeable if the object is moving at a fairly high speed, although it is still theoretically present at any speed.

When an object is moving away from the listener, the sound waves are stretched over a further distance meaning they happen less often. The wavelength ends up being greater so the frequency is less and the pitch is lower. When an object is moving towards the listener, the waves are compressed over a small distance making a very small wavelength and therefore a large frequency and high pitch. Since the pitch of the sound depends on the frequency of the waves, the pitch increases when the object is moving towards the listener.

f′=f(v±v0v∓vs){\displaystyle f'=f({\frac {v\pm v_{0}}{v\mp v_{s}}})}

f' is the observed frequency, f is the actual frequency, v is the speed of sound (v=336+0.6T{\displaystyle v=336+0.6T}) T is temperature in degrees Celsius, v0{\displaystyle v_{0}} is the speed of the observer, and vs{\displaystyle v_{s}} is the speed of the source. If the observer is approaching the source, use the top operator (the +) in the numerator, and if the source is approaching the observer, use the top operator (the -) in the denominator. If the observer is moving away from the source, use the bottom operator (the -) in the numerator, and if the source is moving away from the observer, use the bottom operator (the +) in the denominator.

Ultrasound is sound that has too high a frequency for humans to hear. Some other animals can hear ultrasound though. Dog whistles are an example of ultrasound. We can't hear the sound, but dogs can. Audible sound is in the frequency range between 20 Hz and 20000 Hz. Anything above that is ultrasound, and anything below that is called infrasonic.

Ultrasound also has medical applications. It can be used to generate images with a sonogram. Ultrasound is commonly used to look at fetuses in the womb.

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